I agree that the B8500 is Samsung's best LCD made to date. Its 2D performance still holds its own against top flat panels today. Color accuracy is actually very good, thanks to its CMS. Grayscale and gamma aren't the greatest (only 2 point white balance) but still good. You seem to be pretty aware of the issues the B8500 has (blooming, poor viewing angles etc.). The ST50 will give you a more natural looking picture, better motion resolution, viewing angles and 3D capability. The Blacks won't be quite as good on axis, though (but still VERY good, especially the larger sizes).
FYI, in case you didn't know, the larger sized Panasonic Plasmas (60"+) are supposed to have better black levels, just like last year. So with that said, if I were you, I think the 60ST50 would be a worthy upgrade and/or a 55/65VT50. Personally, if I had a 55B8500, I wouldn't consider an ST50 to be a worthy upgrade unless you were going bigger, or the issues you're having with the B8500 bother you enough, and/or you're interested in 3D.
That's just my 0.02 and what I would do in your position.